1. Technical Field
The present application relates generally to an improved data processing system and method. More specifically, the present application is directed at providing a means to transmit serial attached SCSI out-of-band primitives over an optical interface.
2. Description of Related Art
Small computer system interface (SCSI) is a hardware interface that allows for the connection of up to fifteen peripheral devices to a SCSI host adapter. SCSI uses a bus structure and functions like a mini-network connecting sixteen devices, with the host adapter counting as one device. SCSI allows any two devices to communicate at one time.
Serial attached SCSI (SAS) is emerging as the next adopted high end hard disk drive (HDD). SAS is a serial version of the SCSI interface, which has been parallel since its inception in 1986. SAS is a point-to-point architecture that uses a host bus adapter with four or more channels that operate simultaneously. SAS has several benefits over currently accepted Fibre Channel HDDs, such as including support for existing SCSI commands, larger configuration sizes theoretically reaching 16,384 devices, support for serial ATA (SATA) built into the protocol, and out-of-band (OOB) signaling.
Out-of-band signals are low-speed signal patterns detected by the physical interface. The patterns do not appear in normal data streams, and are used to reset the link, identify the type of attached devices, and negotiate link rate. The patterns consist of defined amounts of idle time followed by defined amounts of burst time, and are transmitted using the same wires used for primary communication.
Larger configurations of SAS storage devices require cable connections between storage enclosures. A popular solution today with Fibre Channel HDD enclosures is optical small form-factor pluggable transceiver and cables. The small form-factor pluggable (SFP) is a compact optical transceiver used in optical communications for both telecommunication and data communications applications. The SFP interfaces a network device to a fiber optic or unshielded twisted pair networking cable.
Having an optical interconnect has numerous advantages over copper interconnect solutions, including lower electromagnetic emissions, less bulky mechanicals allowing for robust strain relief and easier cable routing, and dramatically improved supported cable lengths supporting kilometers of cable distance versus just a few meters with existing SAS solutions. The problem that exists today is how to pass the out-of-band signaling built into the SAS protocol over the standard optical SFPs available today. Optical SFPs transmit data by turning a laser diode (generically any light source) on and off, which represents the logic level “1” and logic level “0”.
One issue is that the OOB signaling protocol requires a third logic state for a bus IDLE, which is not presently possible with optical SFPs. In copper interconnects the SAS interface is a differential pair. When normal data is being transmitted, one signal is driven high while the other signal in the pair is driven low. The receiver subtracts the signal. As part of the OOB protocol, the two signals of the pair must equal each other to present an IDLE to the receiver. Currently, it is not possible for an optical SFP to transmit the IDLE condition.